In recent years, as a semiconductor light-receiving device for optical fiber communication, several types of semiconductor light-receiving devices are proposed. Especially, a mesa structure semiconductor light-receiving device is attracting attentions. The reason is that it can be produced in mass by simple processes with low cost. And it could also reduce a parasitic capacitance in order to achieve high-speed.
FIG. 10 shows a schematic diagram of a semiconductor light-receiving device having a mesa structure. As shown in FIG. 10, in a semiconductor light-receiving device 900 with a mesa structure, an n electrode 901 is provided at the rear surface of a substrate 902, and a mesa structure is adopted at the light incident side of the substrate 902. And inside of it, an n type cladding layer 903, absorbing layer 904 and p type cladding layer 905 are laminated. Furthermore, over the light incident surface of the p type cladding layer 905, a p electrode 906 is provided. As described above, for the semiconductor light-receiving device with a mesa structure, as a pn junction excluding a light-absorbing unit is etched, a pad electrode capacitance on p side can be reduced. This is an advantage as a light-receiving device used for telecommunications sector for high-speed response.
Moreover, as a semiconductor light-receiving device, there is an Avalanche Photo diode (APD) having a structure with a signal amplifying function inside the device in light of improving sensitivity. FIG. 11 shows a basic structure of a mesa APD. As shown in FIG. 11, an APD 910 adopts a mesa structure on a light incident side of a substrate 912. And an n type cladding layer 913, avalanche multiplication layer 914, electric-field relaxation layer 915, absorbing layer 916, p type cladding layer 917 and p type contact layer 918 are laminated. On a sidewall of the laminated layer, a lateral protection film 919 is formed. Furthermore, to the incident surface of an input light, a p electrode 920 is formed.
As for the absorbing layer 916, a semiconductor material capable of sufficiently absorbing an incident light is selected. And especially for communication, InGaAs is adopted which maintains a high absorption coefficient on shorter wavelength side than the wavelength 1.60 μm. As for the multiplication layer 914, a wide gap material is selected which is able to suppress a leak current even in a high electric field as it speeds up and multiplies the injected carrier. Especially for communication, InAlAs or InP is adopted.
In the APD 910, an electron or positive hole (first carrier) generated by absorbing a light in the absorbing layer 916 is accelerated by an electric field inside the absorbing layer 916 of the APD, which is generated by applying a reverse bias. The first carrier is injected into the avalanche multiplication layer 914 while holding kinetic energy and collides with a neutral atom inside the avalanche multiplication layer 914. As a result, an electron and positive hole (second carrier) are generated. Moreover, the first carrier and second carrier are accelerated by the electric field and by colliding to a neutral atom, a new carrier is generated. By the process consecutively occurring, the generated electron and positive hole are expotentially increased, that is, multiplied. By this, the APD is able to sense a small signal as compared to a normal photodiode.
Considering an APD used in an optical communication band, a InGaAs layer and InAlAs multiplication layer which are lattice matched over an InP substrate are the basic structure. When applying a reverse bias to the pn junction, an internal electric field distribution at an operation of the APD is shown in FIG. 12. An electric field of each layer is controlled by the doping concentration distribution among the multiplication layer, electric-field layer and absorbing layer. The vertical axis of FIG. 12 is an electric field E.
An important point to operate the APD successfully is to control each of the electric fields in the absorbing layer and multiplication layer. For example for the APD used in the abovementioned optical communication band, an electric field of the InGaAs absorbing layer must be controlled from 50 to 150 kV/cm, an electric field of the multiplication layer must be controlled to 600 kV/cm or more. The InGaAs constituting the absorbing layer has a narrow gap and its band gap energy is 0.75 eV. Thus in an electric field of 150 kV/cm or more, a noise caused by a tunneling current is generated, causing a sensitivity deterioration. Moreover, a high electric field more than necessary is not preferable in light of reliability. Because the electron or positive hole generated in the absorbing layer is not accelerated enough in not more than 50 kV/cm, an energy barrier with the adjacent semiconductor layer cannot be overcome by drift running, thus a problem is generated in terms of high-speed response characteristics or the like.
On the other hand in the multiplication layer, the carrier injected into the multiplication layer is accelerated by applying a high electric field and collides with lattice to generate a new pair of electron-positive hole. A signal is amplified by this process repeatedly occurring in the multiplication layer, however to occur the process consecutively, an electric field of 600 kV/cm or more is required.
As in the abovementioned example, the most important thing in the APD is to control the electric fields of the avalanche multiplication layer and absorbing layer. In order to make the APD operate properly, optimum electric fields are required for the absorbing and multiplication layers. The control of the electric field distribution is performed by controlling the layer thickness of the electric-field relaxation layer held between the avalanche multiplication and absorbing layer and carrier concentration. That is, controlling the width of the electric-field relaxation layer and carrier concentration is an important key to the reliability and characteristics of the APD.
In order to achieve an APD having a high reliability in a conventional method, a planar structure or pseudo planar structure is adopted for controlling the electric field using an ion implantation and diffusion technique or the like for the abovementioned layer structure to form a pn electrode over a crystal surface (for example non-patent document 1). This method surrounds the multiplication layer, electric-field relaxation layer and absorbing layer with InP implanted with Be ion so as to avoid exposing the multiplication layer portion where a high electric field is applied. Such structure is referred to as a guard ring structure.
Furthermore, in the APD having a planar structure, the periphery of the guard ring structure is protected with a SiNx film. However for the APD having these structures, there are problems that the manufacturing method is generally complicated. And it is difficult to improve characteristics of the device and a tolerance of manufacturing condition.
On the other hand, as a mesa semiconductor light-receiving device, a pn structure is formed in growth process to form a light-receiving area by a mesa etching, and an electric field distribution is distributed one-dimensionally, so the device design is easy. Thus it has advantages of a higher degree of freedom, easier to improve device characteristics and improve manufacturing yield as compared with the abovementioned planar type. However a device that achieved sufficient reliability has not developed yet, because a high electric field is applied to the absorbing layer and avalanche multiplication layer and in addition these layers are exposed to the surface.
Next, characteristics of a device are considered from device fabrication. In the mesa semiconductor light-receiving device, as shown in FIG. 11, the sidewalls of the multiplication layer 914 and field electric relaxation layer 915 which are applied with a high electric field and the absorbing layer 916 that is narrow gap and likely to generate a tunneling current are exposed. In light of device reliability, it is important to protect the lateral faces of these layers.
As a conventional method, a method for obtaining a protection film with a SiNx film created by a plasma CVD (Chemical Vapor Deposition) method or a method for protecting with polyimide and BCB or the like are suggested. However in these protection films, there is a problem in stability of the semiconductor and interface. Furthermore, for the polyimide and BCB or the like, there is also a problem in hygroscopicity. Therefore, as for the APD constituted of a mesa structure, it has been difficult to achieve a high reliability of more than million hours in an operation of the APD at 85 degrees.
In a normal mesa type photodiode, a SiNx film created by a plasma CVD is used as a protection film. The abovementioned SiNx protection film is manufactured by a plasma CVD using SiH4 and NH3 as materials. This method has extremely simple processes and is advantageous in terms of reproducibility and process cost.
However when applying the abovementioned method to the APD, hydrogen radicals diffuse into the APD which are generated by the decomposition of the NH3. Especially for the hydrogen diffused in the electric-field relaxation layer, the hydrogen radical stably bonds with main components of the electric-field relaxation layer in the vicinity of an impurity, and the impurity becomes into a state not satisfactory functioning as an acceptor. And after a passivation, the carrier concentration of the electric-field relaxation layer changes especially near the sidewall.
This increases a leak current of the side wall thus it is not a preferable method in light of device characteristics and reliability. Furthermore as described above, the carrier concentration of the electric-field relaxation layer is a key to the reliability of the APD and if the carrier concentration changes, it brings a cause to deteriorate the reliability of the APD. However it is extremely difficult to control the amount of diffusion of the hydrogen radical during the process, thus it is difficult to improve reproducibility and reliability. This has been a problem for structure and manufacture in a mesa APD.
As a method for preventing the influence by the diffusion of the abovementioned hydrogen radical, as a conventional technique, there is a method (for example non-patent document 2) for not etching the multiplication layer to remain, laminating InP doped with Fe in the etched portion (guard ring) and protecting its surface with a SiNx film. Even in this method, as with the abovementioned non-patent document 1, the manufacturing process is extremely complicated and it is difficult to improve device characteristics and yield.
[Non-Patent Document 1]
    Isao Watanabe, Takeshi Nakata, Masayoshi Tsuji, Kikuo Makita, Toshitaka Torikai, and Kencho Taguchi, J. Lightwave Technol., vol. 18, p. 2200-2207, December 2000.[Non-Patent Document 2]    S. Tanaka, S. Fujiki, T. tsuchiya, S. Tsuji, Monday Afternoon, OFC2003, vol. 1, 67